Presence detector

ABSTRACT

Herein is described an apparatus for detecting the presence of a car within a section of railway track. The track section is defined by low impedance electrical connections between rails at opposite ends thereof, and the section is inductively coupled to transmitter and receiver units. In one embodiment, the transmitter unit operates in the audio frequency range and the proximity of a car is detected by a reduction in the received signal due to electromagnetic coupling of the metallic mass of the car to the track section. In another embodiment the transmitter operates in the ultrasonic frequency range and the presence of a car within the section is detected by a reduction in the received signal caused by the shunting effect of the wheels and axle of the car. In a third embodiment the transmitter unit applies a pair of signals at different frequencies, and the proximity of the metallic mass of the car is detected by a variation in the received amplitude of one of the signals, and the shunting effect of the wheels-axle set is detected by a variation in the other signal.

United States Patent Crofts 1 Aug. 8, 1972 [54] PRESENCE DETECTOR [57]ABSTRACT Iflvemo" George B-CWns, 7 Mountain Herein is described anapparatus for detecting the Clafemom, Callf- 91711 presence of a carwithin a section of railway track.

[22] Filed: July 21, 1970 The track section is defined by low impedanceelectri- 21 Appl. No.: 56,807

[52] US. Cl. ..246/40, 246/34 CT [51] Int. Cl. ..B61l 23/30 [58] Fieldof Search ..246/34 CT, 34 R, 122 R, 40,

Primary Examiner-Arthur L. La Point Assistant Examiner-George H. LibmanAttorney-Robert E. Geaugue cal connections between rails at oppositeends thereof, and the section is inductively coupled to transmitter andreceiver units. in one embodiment, the transmitter unit operates in theaudio frequency range and the proximity of a car is detected by areduction in the received signal due to electromagnetic coupling of themetallic mass of the car to. the track section. In another embodimentthe transmitter operates in the ultrasonic frequency range and thepresence of a car within the section is detected by a reduction in thereceived signal caused by the shunting effect of the wheels and axle ofthe car. In a third embodiment the transmitter unit applies a pair ofsignals at different frequencies, and the proximity of the metallic massof the car is detected by a variation in the received amplitude of oneof the signals, and the shunting effect of the wheels-axle set isdetected by a variation in the other signal.

13 Claims, 9 Drawing Figures AMPL/F/El? flMPL lF/Ef? 54b 34a lOSCILAATO/P OSC/LLATO/F Ream V51? 52 c l HI 72. l 458 35 /46 A OurPuTPATENTEDAUB 8 m2 3.683.178 saw u or 4 UWN lqwwk v Nm PRESENCE DETECTORBACKGROUND OF THE INVENTION This invention relates to railway signalingdevices and more particularly to presence detectors for use in railwayclassification yards.

In many railway applications it is necessary to determine whether or nota section of trackis occupied, and for safe efficient operation thedevice that indicates this status must be accurate" and reliable.Heretofore numerous prior art devices have been developed to provide thejust mentioned functionbut these devices have exhibited one or more.serious deficiencies. For example, one type of prior art presencedetector uses a loop of wire enclosing arailroad trackarea in which acar is to be detected. This loop of wire represents a. considerableinstallation and maintenance problem and-is subject to damage bydragging equipment. In some of these prior art systems, the loop of wireis part of the resonant circuit of an oscillator whose frequency ischanged by the inductance variation of the loop when a car is present,and this change in frequency is utilized as an indication of thepresence or absence of a car within the section. It has been found thatsystems which. attempt to base detection upon frequency or phase changesencounter problems in frequency stability which necessitate expensivefrequency drift compensation circuits.

Also in recent years the. length of the wheel base of some cars hasincreased to the point that these cars may straddle the track section(without having a wheel and axle set within the section), and thereforeprior art detector circuits which operate primarily by sensing theshunting effect of car wheels are unreliable. Increasing the detectionsection-of the trackso that a long car cannot straddle the sectionwould. slow the processing of traffic. Further, foreign matter'such asgrease between the wheels and the rails can reduce the reliability ofsystems relying solely uponsensing the-shunting effect of the wheelsacross .the rails. Additionally with some of the longer tank cars, thereis about 3% feet of clearance between the rails and the car bodymakingzproximity detection all the more difficult.

SUMMARY OF THE INVENTION Therefore it is a primary object of the.subject invention to provide a new and improved presence detector whichis economical to install and operate, and which reliably indicates thepresence of a car within arailway track section.

Another object of the subject invention isto provide a presence detectorwhich will operate reliably forlong cars which may straddle thedetection section, as well as standard length cars.

Yet another object is to provide a presence detector which maybeoptimized for detectionof both the proximity of the metallic mass ofthe car, and the shunting effect of a wheel and axle set across therails.

In accordance with one preferred embodiment, the subject inventionrealizes the above mentioned economies of installation and maintenanceby utilizing a short circuited section of track, upon which the presenceof a car is to be detected as the coupling link between transmitter andreceiverunits. These last mentioned units are inductively coupled to thetrack section by coupling inductors disposed between the rails atopposite ends of the track section. The receiver unit senses thereduction of received energy due either to the proximity of the metallicmass of a straddling long car, or to the shunting effect of the wheelsand axle when the wheels of acar are within the section. In anotherpreferred embodiment, the subject invention comprises a dual frequencypresence detector wherein energy at two differentfrequencies isinductively coupled to the above described detection section. The valueof one of the frequencies is selected to optimize the sensing of theproximityof themetallic mass of the car, and the other frequency isoptimized for the detection of theshunting of therails by the wheels ofa car within the section.

BRIEF DESCRIPTION OF THE DRAWINGS The novel features which arecharacteristic of the invention, both as to its organization and methodof construction' and operation, together with further objects andadvantages thereof will be better understood from the followingdescriptiontaken in conjunction with the accompanying drawings in whichthe illustrative embodiments of the invention are disclosed and whereinlike reference numerals indicate like or corresponding parts.

In the drawings:

FIGS. 1 and 2are block diagrams of the presence detector of the subjectinvention installed in association detector of the subject invention.

FIG. 6 is a plan view of the track section and shows air core trackcoupling inductors suitable for use in the apparatusof the subjectinvention.

FIG. 7 is a vertical sectional view taken on a line 7- 7 of FIG. 6.

FIG. 8 is a block and schematic diagram of a dual frequency presencedetector system in accordance with the subject invention.

FIG. 9 is a block diagram of a track network incorporating severalembodiments of the invention for demonstrating some of the applicationsthereof.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring first primarily toFIGS. 1 and 2, the presence detector in accordance with one preferredembodiment of the subject invention is installed in association with ashort circuited section of track where detection of the presence of acar is desired. Rails 10 and 12 are shorted together at the end of thedesired detection area by heavy wire jumper cables 14 and 16, to form acoupling link (loop) between a transmitter track inductor 18 and areceiver track inductor 20. FIG. 1 shows the system connection for astraight section of track, and FIG. 2 depicts the system connection fora track section containing a switch 22. Jumper cables 24 and 26 providelow resistance electrical continuity through the track switch 22.

Alternating current energy from a transmitter unit 28 is inductivelycoupled by means of the transmitter track inductor 18 through the trackloop 10, 12, 14, 16 (indicated generally by reference numeral 30) to thereceiver track inductor 20. The inductor is coupled to a receiver unit32, which unit senses a reduction in received signal and provides acontrol signal indicative thereof when a car is within the tracksection. When the wheels of a car are between the transmitting andreceiving inductors l8 and 20, the wheels and axle shunt the couplingbetween inductors 18 and 20, thereby reducing the signal to the receiver32.

It is possible for a long car to straddle the track section with one setof wheels to the left of the inductor 18 and the other set to the rightof inductor 20, so that the coupling between inductors is not shunted.However in accordance with the subject invention, the metallic mass ofthe car is electro-magnetically coupled to the track loop 30, therebycausing a detectable reduction in the signal at the receiver 32. It hasbeen found that this just described proximity efiect is observable whenthe frequency of the transmitter 28 is at least within the audiofrequency range (100 to 20,000 Hz). However tests indicate thatproximity detection is most efficient at a frequency of approximately500 Hz for the difficult detection case of long cars which straddle thesection with a high clearance (for example 3% feet) between the body ofthe car and the rails. The improved proximity detection efficiency atthe lower frequencies is believed to be due in part to the relativeincrease of the strength of the radiated electro-magnetic field (nearfield) at the lower frequencies.

Reference is now directed primarily to FIG. 3, which shows theembodiment of FIGS. 1 and 2 in greater detail. The transmitter 28includes an oscillator 34, the output signal from which is amplified byan amplifier 36 and then coupled by a transformer 38 to a track couplingcircuit 40. The circuit 40 comprises a capacitor 42 connected in serieswith the inductor 18.

The values of the elements of circuit 40 may be selected so that circuitis tuned to series resonance thereby providing a low impedance andreducing capacitive cable losses when the unit 28 is located asubstantial distance from the track loop 32.

Energy radiated by the inductor 18 is coupled through the loop 30 to theinductor 20, which inductor forms a part of the receiver couplingcircuit 44. Circuit 44 includes a capacitor 46 and may be tuned toseries resonance in a manner similar to that described previously forcircuit 40.

Referring momentarily to FIG. 4, it is noted that for installationswhere long cable runs are not involved, a low current, high impedanceconfiguration of circuits 40 and 44 may be mechanized by connectingcapacitors 42 and 46 in parallel with inductors 18 and 20, respectively.In this configuration the circuit values may be selected such thatcircuits 40 and 44 operate in an anti-resonance (parallel resonance)mode.

Again referring primarily to FIG. 3, the energy induced into inductor 20is coupled by means of circuit 44 and transformer 48 to a band passfilter 50. A potentiometer 52 is connected across the output terminalsof the filter 50 to form a gain control circuit at the input to anamplifier 54. The output signal of amplifier 54 is detected by anamplitude detector circuit 56. The circuit 56 is coupled to a relaydrive circuit 58 and provides an unoccupied signal thereto whenever theinput signal to the detector 56 is above a predetermined level relativeto a quiescent or reference level. In response to the unoccupied signal,the relay driver circuit energizes the relay 42 (the position shown inFIG. 3) thereby indicating at the control output terminals that a car isnot in the track section. When a car is present in the track section 30,the signal coupled to receiving coupling circuit 44 is reduced below thepreselected threshold level, the unoccupied signal" is removed, relaydrive circuit 58 is de-energized, and the relay switches to an openposition indicative of the presence of a car within the section 30.

One type of track coupling inductor suitable for elements l8 and 20 isshown in FIG. 5 as comprising a laminated core 62 with a coil 64 wrappedon the central position of the core. In one configuration which hasproven satisfactory, the core 62 had the approximate dimension of 1 inchby 1 inch by 20 inches. Inductors 18 and 20 may be buried approximatelyequidistant between the rails, with the longitudinal axis of the coilvertical. Another type of track coupling inductor suitable for theelements 18 and 20 is shown in FIGs. 6 and 7 as an air core inductorwith vertical axes. This last mentioned inductor may be formed by aconductor 66 coiled in a circular pattern, and may be situated on theroadbed approximately equidistant between the rails.

As discussed previously, a frequency of approximately 500 Hz was foundto be optimum for proximity detection of long cars with approximately 3%feet clearance between the car body and the rails. However it has alsobeen noted that systems which operate at low frequencies require a lowshunting resistance to reliably detect the wheels of a car within thesection. This is due to the reduction in inductive impedance of therails at the lower frequencies. Since it is quite common to encounterforeign matter such as grease and oil on the rails (especially inclassification yards), sometimes the shunting resistance is not lowenough to allow detection of a single wheel-axle set at the lowerfrequencies. At the higher frequencies, for example 50,000 to 1,000,000[-12, shunting sensitivity is such that the presence of car wheels maybe readily detected even with oil on the rails. However the higherfrequency systems encounter reduced sensitivity for proximity detection.

The dual frequency presence detector system shown in FIG. 8 uses both alow frequency to optimum proximity detection, and a high frequency forreliable shunt detection. Referring now primarily to FIG. 8, thetransmitter unit 280 comprises oscillator 34a, amplifier 36a, andtransformer 38a, and provides alternating energy at a low frequency.Oscillator 34b, amplifier 36b and transformer 38b provide the highfrequency energy. The secondary windings of transformers 38a and 38b areconnected in series to drive one two-wire cable which is connected totrack inductor circuit 40c. The secondary winding of the high frequencytransformer 38b exhibits a low impedance to the lower frequency andhence readily passes that signal. The secondary winding of the lowfrequency transformer exhibits a high impedance to the higher frequency,and is therefore byepassed for the high frequency signals by a capacitor70 which has negligible effect upon the low frequency signal current.

The track coupling circuit 40c may be constructed in a similar manner tothe circuit 40 described previously. In the embodiment of FIG. 8, thecapacitors 42a and 42b are connected in association with the low andhigh frequency inductors 18a and 18b respectively, so that the circuit40c exhibits series resonance characteristics.

The high frequency inductor 18b may be constructed similar to the lowfrequency inductor described previously (FIG. 5) except that the core iscomposed of a material (such as a ferrite) suitable for high frequencyoperation, and a relatively smaller number of turns are used for thecoil.

The high frequency tuning capacitor 42b exhibits a high impedance at lowfrequencies, and the low frequency inductor 18a has a high reactance atthe higher frequencies so the inductor circuits may be conducted inparallel to the common cable with negligible interaction.

Similarly the receiver unit 32c has dual channels with the componentsfollowed by the letter 0 indicating the low frequency circuits, andthose followed by the letter b, the high frequency circuits. A capacitor72 is added across the primary winding of the low frequency transformer48a to by-pass the high frequency signal around this low frequencyprimary winding.

The output signal of the low and high frequency amplitude detectorcircuits 58a and 58b are combined in a detector logic circuit 74, whichmay be for example an AND gate, which provides a combined unoccupiedsignal if, and only if, an unoccupied signal is supplied thereto fromboth detectors 58a and 58b. The combined unoccupied signal energizesrelay driver circuit 58 when the absence of a car in the section isverified by both the high frequency and low frequency channels. Ifeither amplitude detector does not apply an unoccupied signal," therelay 60 is de-energized indicating that a car is within the section.

In the operation of the dual frequency presence detector of FIG. 8, alow frequency signal at approximately 500 Hz, for example, is applied.to the loop 30 through the transmitter channel comprising the elements34a,-36a, 38a, 42a and 18a. This signal is then coupled through the loopto the receiver inductor circuit 44c, and is processed by the elements48a, 50a, 52a, 54a and 56a. If the low frequency signal applied to theamplitude detector 560 is below a preselected threshold value, theunoccupied signal is not generated by this last mentioned unit andcircuit 74 does not energize the relay driver circuit 58, therebyindicating that the proximity of the metallic mass of a car has beendetected within the section.

Similarly the high frequency transmitter and receiver channel indicatedby the letter b processes a high frequency signal and if the wheels of acar shunt the rails within the section, the signal to amplitude detector56b falls below a preselected threshold level, and an unoccupied signalis not applied to the circuit 74.

The absence of thislast mentioned signal causes relay driver unit 58 tobe de-energized, indicating that the presence of a car has been detectedwithin the section due to the shunting effect of its wheels.

FIG. 9 illustrates several applications of the above describedembodiments of the invention in association with a track networkcomprising rails through 85. Rails 82, 83 and 84, 85 cross rails 80, 81to form short track sections at the areas of intersection designated byreference numerals 88 and 90 respectively. A third track section 94 isformed by the portion of the rails 80, 81 between rail 82 and coupling96. It should be noted that although the embodiments of FIGS. 1 and 2depict track sections defined by a pair of jumper cables 14, 16 acrossthe rails, the invention is readily adaptable to sections defined by anypair of low impedance paths between the rails. For example, one end ofsection 94 is formed by the short across rails 80, 81 provided byintersecting track 82, and the other end by coupling 96, which includesa capacitor 98. The value of capacitor 98 is selected so that thecoupling is a low impedance at the frequency of operation of theassociated detection system, for example, capacitor 98 may be 100,000microfarads.

Track section 94 which could exceed 30 feet in length, is monitored by adual frequency detector system such as described relative to FIG. 8. InFIG. 9, receiver 320 and transmitter 28c are coupled to associated trackcoupling inductors 18 and 20 respectively, by composite leads (twoconductors per lead) such as lead 100.

The length of track section 88, sometimes referred to as a dead section,is too short for effective proximity detection and so a high frequencywheel-axle shunt detecting system is employed. The detection systemassociated with section 88 comprises a high frequency transmitter 102and a high frequency receiver 104 with the associated coupling inductors106 and 108 disposed in opposite corners of section 88. Receiver 104,transmitter 102 and inductors 106, 108 may be constructed in accordancewith the description of the high frequency channel of FIG. 8.

A significant advantage is obtained by locating the inductors 106 and108 in opposite corners of the section, whereby the presence of a car isdetected irregardless of whether it is traveling on rails 82, 83 or 80,81.

Assuming that section 92, which may be 15 feet in length, for example,is too short for effective proximity detection, then only high frequencyshunt detection would be utilized. As shown in FIG. 9, inductors 112 and116, transmitter 114 and receiver are disposed in the section 92 in asimilar manner to that described for section 88, and these units may beidentical to the corresponding units of section 88.

Also although not shown in FIG. 9, the section 90 could be monitored inan identical manner to section 88.

It is noted that in the interest of clarity, the receiver unit 32 hasbeen shown in block diagram form as including the amplitude detector andrelay circuits in ad- I dition to the coupling circuit, filter andamplifier elements. However it 'will be readily recognized that theelements 44, 48, 50, 52 and 54 could be considered receiving means,while elements 56, 58 and 60 could be identified as detection means.

Having thus described a new and novel presence detector which is highlyreliable in operation, and economical to manufacture, install andmaintain, what is claimed is:

1. An apparatus for detecting the presence of a car within a section ofa railway track defined by low impedance electrical connections acrossthe rails at opposite ends of the track section, said apparatusincluding:

transmitter means including a pair of oscillators for inductivelycoupling electrical energy at first and second frequencies to said tracksection; receiver means inductively coupled to said track section forproviding first and second electrical signals representative of theelectrical energy at said first and second frequencies, respectively,coupled to said receiver means from track section;

detection means, coupled to said receiver means, for

detecting variations in said first and second electrical signals;

means for indicating the presence of a car within said track section asa function of the variations in said first and second electricalsignals;

a pair of transmitter track coupling inductor circuits;

and

means for applying the output signals from said pair of oscillators inan electrical parallel arrangement to said pair of transmitter couplinginductor circuits.

2. The apparatus of claim 1 wherein each of said transmitter trackcoupling inductor circuits including separate circuit means foroperating each of said inductor circuits at approximately seriesresonance for a different one of said frequencies.

3. The apparatus of claim 1 wherein said means for applying the outputsignals includes a pair of transformers with each transformer having aprimary winding excited by energy at a different one of saidfrequencies,

and a secondary winding series coupled with the secondary winding of theother transformer.

4. The apparatus of claim 3 wherein one of said secondary windings isshunted by a capacitor.

5. The apparatus of claim 1 further including a pair of receiver trackcoupling inductor circuits; and wherein said receiver means includingmeans for applying the output signals from each of said receiver trackcoupling inductor circuits in an electrical parallel arrangement toseparate circuits for providing the first and second electrical signals.

6. The apparatus of claim 1 including a pair of receiver track couplinginductor circuits; and wherein said receiver means including means forapplying the output signals from each of said receiver track couplinginductor circuits in an electrical parallel arrangement to separatecircuits for providing the first and second electrical signals.

I 7. An apparatus for detecting the presence of a car within a sectionof railway track, said apparatus including:

transmitter means for coupling electrical energy at first and secondfrequencies to said track section;

receiver means for providing first and second electrical signalsrepresentative of the electrical energy at said first and secondfrequencies, respectively, coupled to said receiver means from saidtrack section;

track coupling inductor circuits, said transmitter and said receivermeans being inductively coupled to said track section by different trackcoupling in- 3iilit mg 3% 3l",'lir3?i5%'ci%s disposed at opposite endsections of said track section substantially equidistant between therails of said track section;

detector means, responsive to said first and second electrical signalsfor detecting variations in said first and second signals; and,

means, coupled to said detector means, for indicating the presence of acar within said section as a function of variations in either said firstor second electrical signals.

8. The apparatus of claim 7 wherein said transmitter means includesmeans for generating said first frequency at a frequency below 20,000 Hzand said second frequency at a frequency above 50,000 Hz.

9. The apparatus of claim 8 wherein said coupling inductor circuits eachincluding a separate coil wrapped metallic core inductor located in theroadbed at opposite ends of said track section.

10. The apparatus of claim 8 wherein said coupling inductor circuitseach including a separate air core inductor positioned approximatelyequidistant between the rails within said track section.

11. The apparatus of claim 8 wherein said track coupling inductorcircuits each including means for operating at approximately a seriesresonance condition.

12. The apparatus of claim 8 wherein said track coupling inductorcircuits each including means for operating at approximately a paralleltuned antiresonance condition.

13. The device of claim 8 wherein the track section includes a trackswitch and further comprises connection means for providing lowimpedance electrical continuity through the track switch.

l I i II!

1. An apparatus for detecting the presence of a car within a section ofa railway track defined by low impedance electrical connections acrossthe rails at opposite ends of the track section, said apparatusincluding: transmitter means including a pair of oscillators forinductively coupling electrical energy at first and second frequenciesto said track section; receiver means inductively coupled to said tracksection for providing first and second electrical signals representativeof the electrical energy at said first and second frequencies,respectively, coupled to said receiver means from track section;detection means, coupled to said receiver means, for detectingvariations in said first and second electrical signals; means forindicating the presence of a car within said track section as a functionof the variations in said first and second electrical signals; a pair oftransmitter track coupling inductor circuits; and means for applying theoutput signals from said pair of oscillators in an electrical parallelarrangement to said pair of transmitter coupling inductor circuits. 2.The apparatus of claim 1 wherein each of said transmitter track couplinginductor circuits including separate circuit means for operating each ofsaid inductor circuits at approximately series resonance for a differentone of said frequencies.
 3. The apparatus of claim 1 wherein said meansfor applying the output signals includes a pair of transformers witheach transformer having a primary winding excited by energy at adifferent one of said frequencies, and a secondary winding seriescoupled with the secondary winding of the other transformer.
 4. Theapparatus of claim 3 wherein one of said secondary windings is shuntedby a capacitor.
 5. The apparatus of claim 1 further including a pair ofreceiver track coupling inductor circuits; and wherein said receivermeans including means for applying the output signals from each of saidreceiver track coupling inductor circuits in an electrical parallElarrangement to separate circuits for providing the first and secondelectrical signals.
 6. The apparatus of claim 1 including a pair ofreceiver track coupling inductor circuits; and wherein said receivermeans including means for applying the output signals from each of saidreceiver track coupling inductor circuits in an electrical parallelarrangement to separate circuits for providing the first and secondelectrical signals.
 7. An apparatus for detecting the presence of a carwithin a section of railway track, said apparatus including: transmittermeans for coupling electrical energy at first and second frequencies tosaid track section; receiver means for providing first and secondelectrical signals representative of the electrical energy at said firstand second frequencies, respectively, coupled to said receiver meansfrom said track section; track coupling inductor circuits, saidtransmitter and said receiver means being inductively coupled to saidtrack section by different track coupling inductor circuits, said trackcoupling inductor circuits each including a different pair of inductorsdisposed at opposite end sections of said track section substantiallyequidistant between the rails of said track section; detector means,responsive to said first and second electrical signals for detectingvariations in said first and second signals; and, means, coupled to saiddetector means, for indicating the presence of a car within said sectionas a function of variations in either said first or second electricalsignals.
 8. The apparatus of claim 7 wherein said transmitter meansincludes means for generating said first frequency at a frequency below20,000 Hz and said second frequency at a frequency above 50,000 Hz. 9.The apparatus of claim 8 wherein said coupling inductor circuits eachincluding a separate coil wrapped metallic core inductor located in theroadbed at opposite ends of said track section.
 10. The apparatus ofclaim 8 wherein said coupling inductor circuits each including aseparate air core inductor positioned approximately equidistant betweenthe rails within said track section.
 11. The apparatus of claim 8wherein said track coupling inductor circuits each including means foroperating at approximately a series resonance condition.
 12. Theapparatus of claim 8 wherein said track coupling inductor circuits eachincluding means for operating at approximately a parallel tunedanti-resonance condition.
 13. The device of claim 8 wherein the tracksection includes a track switch and further comprises connection meansfor providing low impedance electrical continuity through the trackswitch.